> Any modern Java compiler will generate identical bytecode for both. Try it yourself and see!

You may be misunderstanding something here.

If you follow the varargs-style recommendation, then concatenation occurs in the log class.

If you follow the guard-style recommendation, then the interpolated expressions will not be evaluated unless the log level matches.

In the naive approach, concatenation always occurs and all expressions which are part of the interpolation will be evaluated no matter the log level.

Could it be that you were thinking about StringBuffer vs. concatenation, an entirely unrelated problem?